Stroke results in significant morbidity and mortality. The only proven therapy for acute stroke is the thrombolytic agent, rtPA, which must be given within three hours of symptom onset. Hu23F2G is an antibody against receptors on white blood cells which has shown promise in reducing the inflammation occurring after stroke and reducing brain damage in laboratory acute stroke models. It has also proven safe and tolerable in phase 1 and phase 2 trials in the dose to be used in this study. The primary objective of this phase 3 clinical trial is to demonstrate the efficacy of Hu23F2G by showing an improvement in functional outcome (independence) at month 3 between treatment groups in patients with ischemic (non-hemorrhagic) stroke receiving medication up to 12 hours after symptom onset. Secondary objectives include examining the relationship between Hu23F2G and Barthel Index at month 3, examining the relationship between Hu23F2G and survival, and measuring the effect of Hu23F2G on quality of life. This is a multicenter, randomized, double-blind, placebo-controlled study in patients with acute ischemic stroke presenting within 12 hours of symptom onset. Randomization will be stratified according to elapsed time from stroke symptom onset (<6 hours or >6 and <12 hours). In the first part of the study, patients will be randomized to one of 3 treatment groups:Hu23F2G 1.5 mg/kg, Hu23F2G 1.5 mg/kg X2 doses 60 hours apart, or placebo. In the second part of the study, based upon an interim efficacy analysis, one of the two active treatment arms will be discontinued and the study continued as a two treatment arm, double-blind, randomized trial. Approximately 800 total patients will be randomized. Assessments of safety, neurological function and disability, and medical resource utilization will be performed during the screen, treatment, and follow-up segments.